A three-way catalyst removes the three noxious substances CO, HC and NOx from exhaust gas of an engine most effectively when the air oversupply ratio 1 lies within a predetermined range (known as the catalyst window) centered on .lambda.=1.0. This air oversupply ratio 1 is directly related to the air-fuel ratio (hereinafter referred to as AFR) of the fuel mixture supplied to the engine and .lambda.=1.0 is obtained when AFR is equal to the theoretical AFR. In one emission control system known in the art, therefore, an O.sub.2 sensor detecting whether the oxygen content of the exhaust gas is more or less than the theoretical AFR equivalent is provided in the exhaust pipe of an engine, and AFR is controlled in the vicinity of the theoretical AFR by adjusting the fuel supply amount based on the feedback correction coefficient a which is determined from the O.sub.2 sensor outputs.
This system works effectively as far as the engine comprises only one cylinder bank.
However, in engines having a plurality of cylinder banks such as V-type engines or horizontal opposed engines, the fuel mixture is distributed to each cylinder bank via an branched path, and the exhaust from each bank is collected via an branched path to the exhaust pipe and hence to the outside via the three-way catalyst.
In such an engine, if feedback correction of the AFR is performed based on an O.sub.2 sensor provided near to the catalyst in the exhaust pipe, it is possible that there will still be some scatter of AFR between banks. In other words, even if the AFR detected by the sensor is in the vicinity of the theoretical AFR, there is still a possibility that the AFR of one bank is rich, while the AFR of another bank is lean. This kind of phenomenon occurs when, for example, the dimensions of the intake manifolds and fuel injection characteristics are different for different banks. Such scatter of AFR increases, however, the noxious substances in the exhaust gas.
In order to prevent scatter of AFR between banks, Tokkai Sho 62-63156 published by the Japanese Patent Office discloses an AFR controller wherein an O.sub.2 sensor is provided for each bank, and the fuel supply amount is corrected independently for each bank such that the rich time of the AFR detected by these sensors is the same for two banks.
However, it is known that the conversion efficiency of the three-way catalyst actually decreases when the AFR for each bank is stable and the oxygen content of the exhaust gas around the catalyst does not deviate from the theoretical AFR equivalent. This is because the three-way catalyst has both oxidizing and reducing actions and both actions are more effective when the oxygen content of the exhaust gas fluctuates in the vicinity of the theoretical AFR equivalent. In other words, the conversion efficiency of the catalyst is higher when the AFR alternates between rich and lean.